[ruby-core:106027] [Ruby master Bug#18296] Custom exception formatting should override `Exception#full_message`.

Issue #18296 has been updated by Eregon (Benoit Daloze).


mame (Yusuke Endoh) wrote in #note-6:
> Does this proposal include that the ruby interpreter should use `#full_message` to show the error information? This is an incompatibility, is it acceptable?

Yes, let's fix that.
I don't think there is much if any compatibility issue here.
The output of the uncaught exception handler is already the same as the default Exception#full_message AFAIK, let's actually call it.
TruffleRuby already calls `exc.full_message` for the uncaught exception handler.

If the custom `exc.full_message` raises an exception, then it's best to report that exception *and* the original exception using the default `Exception#full_message` (written in C).
This is the current TruffleRuby output for that case and I think it's clear:
```
$ ruby -e 'class Foo < Exception; def full_message(**); raise "bar"; end; end; raise Foo, "message"'
Error while formatting Ruby exception:
-e:1:in `full_message': bar (RuntimeError)
	from <internal:core> core/truffle/exception_operations.rb:183:in `get_formatted_backtrace'
Original Ruby exception:
-e:1:in `<main>': message (Foo)
```

> There is an ecosystem to handle uncaught exception, including application monitoring services (Newrelic, DataDog, ScoutAPM, ...) and Rails' exception handling system for development mode.
> They also should change their code base to use `#full_message` (or more dedicated display depending on their context). I think we need to coordinate them if we introduce this incompatibility. cc @ivoanjo

It's a fair point.
I think for them it can make sense in some cases to not include the backtrace in it or to query it separately for convenience.
Thus I propose adding a keyword argument `backtrace:` to `full_message` so `exc.full_message(backtrace: false)` is like `message` but with all the extra decorations from `did_you_mean/error_highlight`/etc (and not include the backtrace or causes in that case).

----------------------------------------
Bug #18296: Custom exception formatting should override `Exception#full_message`.
https://bugs.ruby-lang.org/issues/18296#change-94613

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
After discussing with @eregon, we came to the conclusion that the current implementation of `did_you_mean` and `error_highlighter` could avoid many issues by using `Exception#full_message`.

We propose to introduce a more nuanced interface:

```ruby
class Exception
  def full_message(highlight: bool, order: [:top or :bottom], **options)
    # ...
  end
end

module DidYouMean
  class Formatter
    def full_message(highlight:, did_you_mean: true, **options)
      buffer = super(highlight: highlight, **options).dup
      buffer << "extra stuff"
    end
  end
end

module ErrorHighlight
  class Formatter
    def full_message(highlight:, error_highlight: true, **options)
      # same as above
    end
  end
end
```



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